I tried to emerge firefox (101.0.1) and got the following results

These are the packages that would be merged, in order:

Calculating dependencies... done!

!!! All ebuilds that could satisfy ">=media-libs/harfbuzz-2.8.1:0=" have been masked.
!!! One of the following masked packages is required to complete your request:

• media-libs/harfbuzz-2.8.2::media-kit (masked by: missing keyword)

(dependency required by "www-client/firefox-101.0.1::browser-kit[system-harfbuzz]" [ebuild])
(dependency required by "www-client/firefox" [argument])
For more information, see the MASKED PACKAGES section in the emerge
man page or refer to the Gentoo Handbook.

I'm no stranger to seeing missing keyword on my gentoo system (normally just ~amd64 because a package isn't marked stable yet).
But this isn't even telling me what keyword is missing, so I don't know how to resolve it.

I'm using firefox-beta-bin-102 etc. without issue, but I'd like to be able to actually build it.